Others try to argue that Jack is over 300 years old, compared to Hiccup who is only 15/18-19/20/21 years old. This is a popular trope in fiction pairing an immortal with a mortal (as an immortal vampire with a simple mortal human), the tragedy of their separation through the test of time a prefereabl tool for drama and tragedy.

Fanon

Hijack is part of the Rise of the Brave Tangled Dragons crossover, where the two are part of the Big 4 along with Rapunzel from Tangled and Merida from Brave, and the only slash ship of the Big 4. They are also part of the smaller, How to Train Your Guardian crossover. It is the most popular slash ship for both characters. On AO3, it is the most written ship for Jack and the second for Hiccup, behind Hiccstrid.

As Jack Frost originates from Nordic myths and Hiccup is a Viking, it is possible for Hiccup to believe in Jack. In addition, the large amount of snow in Berk could be due to Jack visiting the island frequently. This could lead to Hiccup meeting Jack, as Jack is a spirit and can only be seen by those who believe in him.

As a main Big 4 pairing, it is common in many AU scenarios. When cast in roles, Hiccup is typically cast in the role of a female character, although this can vary depending on the AU and fan. One example is the Miraculous Ladybug AU, where Hiccup is cast as a male Ladybug and Jack is cast as Cat Noir. In a WALL-E AU, Hiccup takes the role of WALL-E and Jack takes the role of EVE.

There have also been times when the ship has Jack as the Light Fury's rider or human trusting friend. Since both Hiccup and Toothless have green eyes, while Jack and the unnamed Light Fury's eyes are blue.